Vector transient reflow of lead free solder for controlling substrate warpage

ABSTRACT

A system and method for reflowing lead-free solder to interconnect a plurality of electronic components to a substrate is disclosed. The system includes an oven for preheating the substrate and the plurality of electronic components disposed thereon, and a supplemental heat source disposed in the oven for providing additional heat energy to reflow the solder.

TECHNICAL FIELD

The present invention relates to a system and method for reflowinglead-free solder to electrically connect electronic components to asubstrate.

BACKGROUND OF THE INVENTION

It is well known in the art to mount electronic components to rigid andprinted circuit boards. Typically, solder paste is applied to conductorpad regions on the rigid or substrate. Components are then placed withtheir terminals contacting the solder paste in the pad regions. Thesubstrate is then exposed to relatively high temperatures to activatethe solder paste which melts and then solidifies to bond andelectrically connect the components onto the substrate.

A technique for mounting components onto flexible polyester substrateswith low softening temperatures is taught by Annable in U.S. Pat. No.5,898,992. The flexible substrate is fixed to a carrier support member.A cover is placed over the substrate. The cover has openingscorresponding to component locations and with the carrier forms acarrier assembly. Solder paste is applied to the conductor regions ofthe substrate having component pads. Electronic components are thenplaced on the substrate with their terminals in contact with the solderpaste. The carrier assembly is then pre-heated in a reflow oven to atemperature below the melting point of the solder paste. The assembly isthen subjected to a rapid rise in temperature utilizing a supplementalheat source such as a heated gas jet. The cover shields the substratefrom the high reflow temperatures and minimizes distortion of theflexible substrate during reflow.

While the prior art teaching achieves its intended purpose significantimprovements are needed. For example, it would be desirable to eliminatethe need for a special cover for shielding specific areas of thesubstrate from the heat generated by the gas jet. Additionally, aprocess for reflowing lead-free solder disposed between a substrate anda plurality of electronic components thereon is not addressed by theprior art and is therefore needed.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the present invention a system for reflowing solderto interconnect a plurality of electronic components to a substrate isprovided. The system includes an oven for preheating the substrate andthe plurality of electronic components disposed thereon, and asupplemental heat source positioned within the oven for providingadditional heat energy to reflow the solder.

In another embodiment of the present invention the supplemental heatsource is a nozzle positioned within the oven, wherein the nozzle has aplurality of vanes for directing hot gas transversely across thesubstrate.

Preferably, the circuit conductors on the substrate are copper. Selectedregions of the conductors referred to as component pads are providedwith a surface finish such as tin or immersion silver to enhance theease of soldering to the pads. The spaces between the conductor regionsof the substrate may be filled with electrically isolated regions ofcopper having the same thickness as the conductor regions. These copperareas further shield the substrate during reflow by selectivelyabsorbing heat during the reflow process.

The substrate may be comprised of more than two layers of circuitconductors, commonly referred to as multi-layer circuits. For thesecircuits, two or more layers of the substrate are used and bondedtogether with a suitable adhesive to form four or more conductor layers.

In still another embodiment of the present invention a supplemental heatsource used to activate the solder paste may be supplied by one or morejets of hot gas which are directed toward the exposed areas of thesubstrate. Suitably, the jet of hot gas extends transversely over thewidth of the substrate.

In yet another embodiment of the present invention, a method forreflowing solder to interconnect a plurality of electronic components toa substrate is provided. The method includes inserting the substrateinto an oven, preheating the substrate and the plurality of electroniccomponents disposed thereon, providing additional heat energy to reflowthe solder using a supplemental heat source positioned within the oven.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus for reflowingsolder to electrically connect electronic components to a substratemounted on a pallet, in accordance with the present invention;

FIGS. 2 a–2 b is a cross-sectional and plan view of a preferredembodiment of a phase-transition pallet, in accordance with the presentinvention;

FIGS. 3 a–3 d are cross-sectional views of the phase-transition pallethaving a substrate on which electronic components are mounted on bothexposed sides of the substrate, in accordance with the presentinvention;

FIGS. 4 a–4 b is a schematic representation of a system for reflowingsolder to electrically connect electronic components to a substrateusing a stencil, in accordance with the present invention;

FIG. 5 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a substrate using an arrayof hot gas nozzles, in accordance with the present invention;

FIG. 6 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a substrate using an infrared light source, in accordance with the present invention;

FIGS. 7 a–7 b is a schematic representation of a system for reflowingsolder to electrically connect electronic components to a substrateusing a protective cover, in accordance with the present invention;

FIG. 8 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a substrate using a pallethaving heat pipes, in accordance with the present invention;

FIG. 9 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a substrate using a pallethaving thermoelectric coolers, in accordance with the present invention;

FIG. 10 is a schematic representation of a system for reflowing solderto electrically connect electronic components to a substrate using a gasnozzle having vanes that direct the gas in a transverse direction, inaccordance with the present invention;

FIG. 11 is a cross-sectional view of the gas nozzle of FIG. 10, havingvanes that direct the gas in a transverse direction, in accordance withthe present invention; and

FIG. 12 is a graph of temperature of a substrate over time in a reflowoven, in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A system 10 for reflowing solder to electrically interconnect electroniccomponents to a flexible, semi-flexible or rigid substrate 12 isillustrated in FIG. 1, in accordance with the present invention.Further, system 10 includes a pallet 20 that provides a means to mountcircuit components on substrate 12 without degrading the materialproperties of the substrate. System 10 additionally includes a reflowoven 13, a conveyor system 16, a gas nozzle 18 and a pallet 20. Thereflow oven has a plurality of heaters 22 to pre-heat the substrate 12to a desired temperature. Conveyor system 16 is configured in aconventional manner to cooperatively receive pallets 14 for movementthrough the reflow oven 13.

Pallet 14 in an embodiment of the present invention is, preferably, aphase-transition pallet 14 for reflowing solder paste to interconnectelectronic components 24 to substrates 12. Phase-transition pallet 14 isconfigured to support substrate 12 and cooperates with conveyor system16 to transport substrate 12 through oven 13. Oven 13's heaters 22pre-heat substrate 12, and hot gas nozzle 18 provides supplementalheating. Solder paste 26 is printed on conductor pads 28 disposed onsubstrate 12 on which components 24 are placed.

Referring now to FIG. 2 a–2 b, an elevation and cross-sectional views ofphase-transition pallet 14 are illustrated, in accordance with thepresent invention. As shown pallet 14 includes at least one internalcavity 40 having therein a phase-change material 42. Support pins 44 areprovided on pallet 14 to hold substrate 12 flat or planar on a palletsurface 46. Pins 44 may be tensioned or loaded by springs 48 to providea tensioning force on substrate 12. In an embodiment of the presentinvention, a picture frame 50 may be used to secure substrate 12 againstpallet surface 46. Picture frame 50, as illustrated attaches to andsecures the periphery of substrate 12 to hold the edges of substrate 12against surface 46 of the pallet.

In another embodiment of the present invention, a phase-transitionpallet 14′ configured to accommodate a double-sided substrate 12 whichelectronic components 24′ are populated on both sides 60, 62 ofsubstrate 12′, is illustrated. In several of the cross-sectional views,as shown in FIGS. 3 a–3 d, pallet 14′ has at least one open cavity 64 toaccommodate electronic components 24′ that have been mounted on thefirst exposed surface 60 of substrate 12′. Open cavity 64 may be filledwith a suitable foam 66, if necessary, to provide additional support forsubstrate 12′.

In a preferred embodiment of the present invention, substrate 12′ is apolyester film having a thickness of 0.003 to 0.010 inches. Copperconductors 68 and solder pads 70 may be formed on both sides 60, 62 ofthe polyester substrate, as is well known in the art. A suitable soldermask (not shown) is applied over copper conductors 68 so that only thepad 70 areas on which solder paste 72 is to be printed are exposed.These pads 70 may have a suitable surface finish such as an organicsurface finish to protect the pad surfaces from oxide formation. Othersurface finishes such as immersion silver or electroplated tin may beused to enhance the solderability of components 24′ to the pads.

Solder pastes 72 that have compositions containing lead, as well assolder pastes having lead-free compositions may be used. The solderpastes containing lead generally have a lower melting temperature ofabout 183° to 200° C., while lead-free solder compositions have meltingtemperatures of about 220° to 245° C.

In operation, as pallet 14 or 14′ having substrate 12 or 12′ affixedthereon is transported through the pre-heat zones in oven, the solderpaste 72 is activated and gradually heated to just below its meltingtemperature. During this process, the phase-transition material 42begins to absorb heat from the oven 13 as well as from the substrate 12or 12′, and thereby lowers the temperature of the substrate. The phasetransition material 42 is selected having a melting point that is lowerthan the melting point of the solder paste 72. As the phase-transitionmaterial 42 begins to melt, the material begins to absorb an amount ofheat or energy equal to the latent heat of the material. Consequently,the temperature of phase-change material 42 is held constant until thematerial is fully melted. Thus, the present invention significantlyenhances the heat absorption properties of the pallet 14 or 14′ andmaintains a lowered substrate 12 or 12′ temperature during reflow of thesolder paste 72.

In a preferred embodiment of the present invention, phase-transitionmaterial 42 exhibits a melting temperature lower than that of solder 72,and may be comprised of conductive metals such as gallium, galliumalloys, or alloys of tin and lead. Other suitable phase transitionmaterials include chloro-fluoro carbons and their compounds.

The supplemental heat created from gas nozzle 18 is utilized to providea focused and concentrated heat source. Gas nozzle 18 provides heat tothe exposed substrate surface for a short duration. The solder paste 26,conductor pads 28, and copper regions of substrate preferable absorbheat because of their high thermal diffusivity, while substrate 12 or12′ is maintained at a lower temperature by the pallet 14 or 14′, whichis held at a lower temperature by the phase-transition material 42. Inthis manner, softening and damage to substrate 12 or 12′ during thereflow process is prevented.

After the exposed region of the substrate passes below gas nozzle 18,the temperature of the exposed electronic component 24 and substrate 12or 12′ rapidly falls so that the activated solder cools and solidifies.A reliable electrical connection between the conductors or pads 20 andcomponents 24 or 24′ is thus formed. During this process, thephase-transition material 42 also solidifies, so that pallet 14 or 14′is ready for reuse.

Referring now to FIGS. 4 a and 4 b, another embodiment of the presentinvention is illustrated wherein a stencil 80 is introduced between thegas nozzle 18′ and the substrate 12 or 12′. The stencil 80 has aplurality of opening or apertures 82 disposed therein. The apertures 82expose certain areas of the substrate 12″ and/or components 24″ to gasnozzle 18′ to reflow the solder paste 72′. The stencil 80 also shieldssubstrate 12″ areas and/or components that are not to be exposed to thegas jet. In this manner solder paste is melted in the appropriate areasand potential damage caused by heating the substrate to elevatedtemperatures is avoided. In another embodiment as shown in FIG. 4 a, thepallet 14″ and stencil 80 are held stationary while the gas nozzletraverses the stencil to selectively heat the substrate areas. Inanother embodiment, as shown in FIG. 4 b, a hot gas nozzle 18″ is heldstationary while pallet 14′″ and stencil 80 move below the gas nozzle18″. This embodiment would require multiple stencils to heat and reflowthe desired areas of the substrate and electronic components.

In yet another embodiment of the present invention, a system 90 forreflowing solder is illustrated in FIG. 5. The present embodimentcontemplates system 90, having an array of gas nozzles 92 positionedabove a conveyor system 94. The array of gas nozzles 92 are computercontrolled and as such may be programmed to separately actuate for adefined period of time. The gas nozzles 92 are programmed to actuate andrelease high temperature gas on selected areas of a populated substrate96 to reflow the solder paste as the components 98 pass underneath thearray of nozzles 91. Preferably, a downward facing camera 100 or opticalscanner is used to read a bar code 102 printed on substrate 96 toidentify the substrate 96 and program the actuation of the array of gasnozzles 91. Array 91 may be constructed from a silicon micromachinedstructure and contain silicon micromachined valves. Other selectiveheating techniques such as soft beam may be used wit the gas nozzlearray 91. Moreover, the present invention contemplates using differentgas pressures in different gas nozzles in the array 91.

In still another embodiment of the present invention, as shown in FIG.6, a system 149 for reflowing solder using an infra-red light source 150as a supplemental heat source 152 is illustrated. In the instantembodiment a substrate 152 is covered with a protective cover 154 thatis impervious to infra-red radiation. Protective cover 154 has aplurality of apertures 156 for exposing the electronic components 158 tobe soldered to substrate 152. The infrared light source 156 may includea plurality of infra red devices to produce a desired heating effect.Further, a Collimating lens 160 is placed between the infra red light150 and the populated substrate 152 to focus the light directed towardthe substrate. Once the protective cover 154 is in place the pallet162/substrate 152/cover 154 assembly is placed on conveyor 16 andtransported through reflow oven 13. The temperature of the oven 13 maybe set at a temperature that does not damage substrate 152. Theadditional heat energy required to reflow the solder paste, disposedbetween the electronic components 158 and the solder pads on thesubstrate is supplied by the infra red light source 150.

FIGS. 7 a–7 b illustrate a protective cover 200 for shielding portionsof the substrate from hot gasses emanating from gas nozzle 202. In anembodiment of the present invention protective cover 200 is made frominsulative materials such as FR4 material or aluminum or the like. Thistype of cover may combined with any of the previous embodiments, asappropriate.

FIG. 8 is a cross-sectional view through a pallet 300. Pallet 300supports a substrate 302 populated with electronic components 304.Pallet 300 includes a plurality of heat pipes 306 which draw heat awayfrom substrate 302 to cooler regions of the pallet. Additionally, theheat pipes are in communication with phase transition regions 310 whichcontain phase transition material, as described previously. The heatpipes and phase transition regions 310 cooperate to cool the substrate302 to insure the substrate is not damage by exposure to the supplementa heat source.

In another embodiment of the present invention, as shown in FIG. 9 a–9b, a pallet 400 is provided having thermoelectric coolers 403 to absorbheat away from substrate 402. As in the previous embodiments asupplemental heat source is applied to substrate 402 populated withelectronic components 404 to reflow solder disposed there between. Asshown in FIG. 9 a, the present invention contemplates an array 408 ofthermoelectric coolers 403 disposed in pallet 400. The array 408 may beindependently actuated and controlled to provide localized cooling.

In still another embodiment of the present invention a transverse flownozzle 500 is provided for directing hot gas transversely across thesubstrate, as illustrated in FIG. 10. Nozzle 500 is generally disposedwithin a reflow oven 502 having conventional heaters or heat sources(not shown). The heaters are located above and below the substrate. Thelower heaters in the oven are maintained at 5 to 15 degrees C. lowertemperature than those on the top. The nozzle may be mounted to apivotable structure (not shown) to allow the nozzle to pivot to directgas upstream or down stream on the substrate. In operation a substrate504 have a plurality of circuit traces attached thereto is carried by aridged pallet 506, such as the pallets disclosed above, into oven 502.Other pallets may be used, such as pallets made of a single materialsuch as Glastik, or made of a composite, such as aluminum for thecontacting surface and an insulator such as FR4 for the back side(bottom surface) of the pallet. The insulator would shield the heat fromthe bottom side, while the aluminum provides heat sinking to the palletand keeps the substrate temperature low. As shown above the pallet wouldhave a cavity to accommodate electronic components attached to the otherside of the substrate. The substrate 504 and pallet 506 are transportedthrough oven 502 via a conveyor belt system 508. The speed of conveyor508 ranges from 10 inches per minute to 50 inches per minute. Typically,substrate 504 will have a plurality of circuit traces (not shown)attached to a top surface 510 of the substrate 504. A plurality ofelectrical components 512, such as surface mount devices are placed ontop of the circuit traces and solder paste (not shown) is disposedbetween the surface mount devices and the circuit traces. The nozzle hasa nozzle exit width d and is positioned a distance (l) from thesubstrate, where the ratio of l/d is less than 14. This ensures that thecentral cone 513 of the hot gas jet, which is about 14 jet diameters (d)long, is intact when the jet impinges on the substrate. Moreover, thisensures improved heat transfer from the jet to the substrate. The hotgas jet is preferably heated air. The substrate may be polyethyleneterephthalate having a glass transition temperature of 85 degree C.

The substrate populated with the electrical components is transportedthrough oven 502 which raises the temperature of the substrate to apredetermined level, preferably to approximately 130° C. Nozzle 500receives hot gas, ranging in temperature from 200 to 500° C., asindicated by arrow (i). The hot gas is distributed over the width of thesubstrate to further heat the components 512, solder paste (not shown),and substrate 504 to approximately 250° C. In this manner the solderpaste is liquefied. The configuration of the nozzle, as well as, thepositioning of the nozzle with respect to the substrate, as will bedescribed below, creates a well defined gas jet or stream 514. Gasstream 514 heats only a desired portion of the substrate leaving otherportions of the substrate unheated. Thus, the present invention preventsdamage to the substrate by focusing the hot gas and exposing onlydesired portions of the substrate to the gas.

FIG. 11 is a cross-sectional view of nozzle 500, in accordance with thepresent invention. Nozzle 500 includes a nozzle housing 520. Nozzlehousing 520 supports a gas distribution pipe 522 at either end of pipe522 via structural support members 524 and 526 and along theintermediate portion of pipe 522 via support brackets 528. Distributionpipe or tube 522 is tapered along its longitudinal axis to facilitateuniform distribution of the hot gas at the nozzle exit and includes aplurality of apertures from which the hot gas emanates. A pair ofgratings 530 and 532 comprised of a wire mesh and/or perforated platesoperate in conjunction with a series of vanes 534 to distribute the hotgas over the substrate. Vanes 534 are positioned at a progressivelyincreasing angle with respect to the vertical, moving from the center ofthe nozzle to the ends of the nozzle. Furthermore, vanes may be curvedto facilitate transverse flow of the gas. Transverse flow is definedherein as, flow directed predominantly perpendicular to the direction oftravel of the substrate through the oven. Further, a pair of deflectors536 having a radius r deflect air down towards the nozzle opening 538.As indicated by arrows (f), hot gas is distributed across a width w ofthe substrate in a transverse direction. Thus, a narrow strip of hot gasis created and impinges only along a desired portion of the substrate.

Thus, the process of reflowing the solder paste between the electroniccomponents and the substrate as described above is controlled bybalancing the temperature within the oven 502, the speed of conveyor508, the temperature of the gas exiting nozzle 500, the gas flow rate,and the width of the exit of nozzle 500 and the distance nozzle 500 isfrom substrate 504. The proper balancing of these parameters, throughthe use of the present invention, provides reflow of the solder pastewithout damaging the substrate.

In a preferred embodiment of the present invention, as shown in FIG. 1,substrate 12 having a plurality of electronic components disposedthereon is placed in a reflow oven 13 having a plurality of heaters 22for preheating substrate 12. The plurality of heaters 22 are preciselycontrolled to provide preheating of substrate 12 prior to exposure tosupplemental heat source 18. The supplemental heat source 18 may be ahot gas nozzle that provides a flow of hot gas over the electroniccomponents 24 positioned on substrate 12 or any other heat energy sourceto reflow solder paste 26 thereon. In the present embodiment, solderpaste 26 is of the lead-free variety having a melting temperature in therange of approximately 217°–220° C.

The present invention contemplates a preferred oven 13 temperatureprofile, as shown in FIG. 12. Specifically, FIG. 12 is a graph oftemperature of the substrate (on the vertical axis) over time (on thehorizontal axis) in the reflow oven. Oven temperature profile is createdby the plurality of heaters 22 as well as supplemental heat source 18 toprovide an overall oven temperature profile 600. Temperature profile600, for example, created by the plurality of an oven heaters 22provides at least two heating levels 602, 604 for gradually increasingthe heat exposure to and temperature of substrate 12. For example, thefirst heating level 602 is reached by controlling a portion of the ovenheaters 22 to increase the temperature of substrate 12 at a rate of notmore than 2° C./second over a first predetermined transitional period606. For example, the first predetermined transitional period is between60 and 120 seconds. The first heating level 602 is reached when thetemperature of substrate 12 is approximately 130°–160° C. The ovenheaters 22 are controlled to hold the temperature of substrate 12 atfirst heating level 602 for a first heating level predefined period 608.For example, the first heating level predefined period is between 120and 240 seconds. After the first predefined heating level period 608 haselapsed, the oven heaters 608 are controlled to provide an increase intemperature of substrate 12 at a rate of not more than 2° C. per second.After a second predetermined transitional period of time 610, substrate12 reaches the second heat level 604 that is approximately in the rangeof 190°–230° C. For example, the second predetermined transitionalperiod is between 30 and 120 seconds. Again, oven heaters 22 arecontrolled to hold the substrate temperature at the second heating levelfor a second heating level predefined period of time 612. For example,the second heating level predefined period is between 60 and 180seconds. After the second heating level period of time 612 has elapsed,the supplemental heat source 18 provides a rapid increase in thetemperature of substrate 12 to a peak temperature 614 of between 230°and 280° C. Further, supplemental heat source 18 is controlled such thatthe duration at which substrate 12 is held at a temperature above 225°C. is not more than 15 seconds.

A preferred system 10 set-up includes a conveyor, transporting thesubstrate 12 at a speed of 40 inches per second. The nozzle temperatureof supplemental heat source 18 preferably is 300° C. System 10 does notin all embodiments require a pallet 14 to support and transportsubstrate 12 on conveyor 16 nor is a pallet required in all embodimentsto dissipate heat from substrate 12. A cover is also not needed toprotect various portions of the electronic assembly. The preferredembodiment of system 10 utilizes oven temperature profile 600 to reflowsolder without damaging substrate 12, rather than using a cover or apallet to control heat exposure to and absorption in substrate 12.

The preferred embodiment of the present invention is uniquely configuredfor reflowing lead-free solder which has a melting temperature between217° and 2200° C. The temperature profile 600 provided by oven 13,heaters 22 and supplemental heat source 18 decreases or eliminates thelikelihood of damage to substrate 12 while providing sufficient heatenergy to reflow the lead free solder.

As any person skilled in the art of system and method for reflowingsolder to electrically connect electronic components to a substratehaving a low softening temperature will recognize from the previousdetailed description and from the figures and claims, modifications andchanges can be made to the preferred embodiments of the inventionwithout departing from the scope of this invention defined in thefollowing claims.

1. A method far reflowing solder to interconnect a plurality ofelectronic components to a substrate, the method comprising: insertingthe substrate into an oven; preheating the substrate and the pluralityof electronic components disposed thereon to a first heating level,wherein the first heating level is maintained for a duration of between120 and 290 seconds; preheating the substrate and the plurality ofelectronic components disposed thereon further to a second heatinglevel, wherein the second heating level is maintained for a duration ofbetween 60 and 180 seconds; heating the substrate to a peak healinglevel to reflow the solder using a supplemental heat source positionedwithin the oven; and positioning the supplemental heat a distance eabove the substrate and providing a supplemental heat source exit havinga dimension d and wherein a ratio e/d is less than or equal to fourteen.2. The method of claim 1 further comprising creating a stream of hot gasusing the supplemental heat source, wherein the gas flows transverselyacross the substrate.
 3. The method of claim 2 wherein inserting thesubstrate into an oven further comprises transporting the substratethrough the oven using a conveyor.
 4. The method of claim 3 whereintransporting the substrate further comprises moving the substratethrough the oven at a speed of between 10 to 50 inches per minute. 5.The method of claim 3 further comprising limiting heat absorption by thesubstrate to prevent damaging the substrate by adjusting a speed of theconveyor, a distance the supplemental heat source is from the substrate,and a temperature of the gas.
 6. The method of claim 1 furthercomprising providing a gas having a temperature range between 200 to 500degrees Celsius.
 7. A method for reflowing solder to interconnect aplurality of electronic components to a substrate, the methodcomprising: inserting the substrate into an oven; preheating thesubstrate and the plurality of electronic components disposed thereon toa first heating level, wherein the first heating level is maintained fora duration of between 120 and 290 seconds; preheating the substrate andthe plurality of electronic components disposed thereon further to asecond heating level; heating the substrate to a peak heating level toreflow the solder using a supplemental heat source positioned within theoven; positioning the supplemental heat a distance e above the substrateand providing a supplemental heat source exit having a dimension d andwherein a ratio e/d is less than or equal to fourteen; and whereinpreheating the substrate further comprises heating the substrate to asecond heating level wherein the second heating level has a duration ofbetween 60 to 180 seconds.
 8. The method of claim 1 wherein the firstheating level is between 130°–160°.
 9. The method of claim 7 wherein thesecond heating level is 190°–230° C.
 10. The method of claim 1 whereinthe peak healing level is between 230°–280° C.